A carotid duplex study is an ultrasound of the carotid arteries, which are found in the neck. It is done in the office by a trained sonographer (ultrasound technologist). The ultrasound uses sound waves to see if there is plaque build up in the arteries. Plaque build up can cause narrowing of the carotid arteries which may decrease blood flow to the brain. Decreased blood flow can cause symptoms such as dizziness, unexplained syncope (passing out), change in sight, or stroke.